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certain addon navigation facilities cause the sim to freeze/crash,
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That's what I was thinking of as a first source of trouble

If you have an AFCAD editor, Ian, you could try and see how ENGM and its surroundings area are built up, navaid-wise.
Then look for either:
- two navaids with the same name, but different frequency
- two navaids with different names, but the same frequency
In these situations the navaids "bite" each other, and this can happen when installing addon scenery, when real-life frequencies (or names) get changed over time, while the original FS9 navaids remain the same, of cause.
As Dominic already said: uninstall the addon scenery, try to recreate the problem, and see if the cause in in there somewhere first. Then, after that, you can apply the AFCAD approach to try and solve the situation...